It is known in the art to use inks, compositions or layers containing oriented magnetic or magnetizable particles or pigment particles, particularly also magnetic optically variable pigment particles, for the production of security elements, e.g. in the field of security documents. Coatings or layers comprising oriented magnetic or magnetizable pigment particles are disclosed for example in U.S. Pat. Nos. 2,570,856; 3,676,273; 3,791,864; 5,630,877 and 5,364,689. Coatings or layers comprising oriented magnetic color-shifting pigment particles, resulting in particularly appealing optical effects, useful for the protection of security documents, have been disclosed in WO 2002/090002 A2 and WO 2005/002866 A1.
Security features, e.g. for security documents, can generally be classified into “covert” security features one the one hand, and “overt” security features on the other hand. The protection provided by covert security features relies on the concept that such features are difficult to detect, typically requiring specialized equipment and knowledge for detection, whereas “overt” security features rely on the concept of being easily detectable with the unaided human senses, e.g. such features may be visible and/or detectable via the tactile senses while still being difficult to produce and/or to copy. However, the effectiveness of overt security features depends to a great extent on their easy recognition as a security feature, because most users, and particularly those having no prior knowledge of the security features of a therewith secured document or item, will only then actually perform a security check based on said security feature if they have actual knowledge of their existence and nature.
A particularly striking optical effect can be achieved if a security feature changes its appearance in view to a change in viewing conditions, such as the viewing angle. Such an effect can e,g. by obtained by dynamic appearance-changing optical devices (DACODs), such as concave, respectively convex Fresnel type reflecting surfaces relying on oriented pigment particles in a hardened coating layer, as disclosed in EP 1 710 756 A1. This document describes one way to obtain a printed image that contains pigment particles or flakes having magnetic properties by aligning the pigment particles in a magnetic field. The pigment particles or flakes, after their alignment in a magnetic field, show a Fresnel structure arrangement, such as a Fresnel reflector. By tilting the image and thereby changing the direction of reflection towards a viewer, the area showing the greatest reflection to the viewer moves according to the alignment of the flakes or pigment particles.
While the Fresnel type reflecting surfaces are flat, they provide the appearance of a concave or convex reflecting hemisphere. Said Fresnel type reflecting surfaces can be produced by exposing a wet coating layer comprising non-isotropically reflecting magnetic or magnetizable pigment particles to the magnetic field of a single dipole magnet, wherein the latter is disposed above, respectively below the plane of the coating layer, as illustrated in FIG. 7B of EP 1 710 756 A1 for a convex orientation. The so-oriented pigment particles are consequently fixed in position and orientation by hardening the coating layer.
One example of such a structure is the so-called “rolling bar” effect (FIG. 1), as disclosed in US 2005/0106367. A “rolling bar” effect is based on pigment particles orientation imitating a curved surface across the coating. The observer sees a specular reflection zone which moves away or towards the observer as the image is tilted. A so-called positive rolling bar comprises pigment particles oriented in a concave fashion (FIG. 2b) and follows a positively curved surface; a positive rolling bar moves with the rotation sense of tilting. A so-called negative rolling bar comprises pigment particles oriented in a convex fashion (FIG. 2a) and follows a negatively curved surface; a negative rolling bar moves against the rotation sense of tilting. A hardened coating comprising pigment particles having an orientation following a concave curvature (positive curve orientation) shows a visual effect characterized by an upward movement of the rolling bar (positive rolling bar) when the support is tilted backwards. The concave curvature refers to the curvature as seen by an observer viewing the hardened coating from the side of the support carrying the hardened coating. A hardened coating comprising pigment particles having an orientation following a convex curvature (negative curve orientation) shows a visual effect characterized by a downward movement of the rolling bar (negative rolling bar) when the support carrying the hardened coating is tilted backwards (i.e. the top of the support moves away from the observer while the bottom of the support moves towards from the observer). This effect is nowadays utilized for a number of security elements on banknotes, such as on the “5” of the 5 Euro banknote or the “100” of the 100 Rand banknote of South Africa.
For optical effect layers printed on a substrate, negative rolling bar effect (orientation of the pigment particles (P) in a convex fashion, curve (V), FIG. 2a) are produced by exposing a wet coating layer to the magnetic field of a magnet disposed on the opposite side of the substrate to the coating layer (FIG. 3a), while positive rolling bar effect (orientation of the pigment particles (P) in a concave fashion, curve (W), FIG. 2b) are produced by exposing a wet coating layer to the magnetic field of a magnet disposed on the same side of the substrate as the coating layer (FIG. 3b). For positive rolling bar, the position of the magnet facing the still wet coating layer may lead to some problems in industrial processes. If the magnet enters in physical contact with the wet coating layer, it may disturb the optical effect layer.
Therefore, a need remains for a method to produce security features displaying a positive rolling bar while avoiding the drawbacks of the prior art.